Fix syntax error of wrong use of self-closing HTML tag

Firefox complaint about this in the source code view:

src/vector/index.html

@@ -72,6 +72,6 @@
         <source src="media/busy.ogg" type="audio/ogg" />
         <source src="media/busy.mp3" type="audio/mpeg" />
     </audio>
-    <audio id="remoteAudio"/>
+    <audio id="remoteAudio"></audio>
   </body>
 </html>